Function
GTPASE CYCLE
GTP-binding proteins such as protein biosynthesis factors, heterotrimeric G-proteins or small Ras-related GTP-binding proteins function as molecular switches cycling between GDP-bound 'OFF'- and GTP-bound 'ON'- states. Exchange of the bound GDP is facilitated by Guanine Nucleotide Exchange Factors (GEFs) which increase the dissociation rate of nucleotides by several orders of magnitude. Once activated, binding of GTP enables them to interact with effectors, which are defined as those proteins binding tightly only to the ON-state.
Hydrolysis of bound GTP is the common timing mechanism to return them to the GDP-bound 'OFF'-state thereby completing what is called the GTPase cycle. GTP-hydrolysis as carried out by these proteins is intrinsically very slow but can be accelerated by orders of magnitude by GTPase Activating Proteins, GAPs, which are specific for the respective GTP-binding proteins.
The importance of proper functioning of the GTPase cycle is demonstrated by the occurrence of diseases associated with either mutations in the GTP-binding proteins, which then function as oncoproteins, or by mutations in GAPs, GEFs and effectors.
COMPLEX OF Gs-ALPHA BOUND TO GTP
THE CYCLE OF G PROTEINS DISSOCIATION AND REASSOCIATION.
Alpha, Beta, and Gamma are the three subunits of the G-protein. The active form is the Alpha-GTP complex (dark teal), while the inactive GDP complexes are shown in light teal. GEF = guanine nucleotide exchange factor ; GAP = GTPase-activating protein.
INTRINSIC GTPASE ACTIVITY
The intrinsic GTPase activity converts the bound GTP to GDP in the a - subunit, returning it to its original conformation. This results in the a - subunit diffusing from the adenylate cyclase and reassociating with the b & g subunits. Adenylate cyclase catalyses the conversion of ATP to cAMP which is then used to activate another, initially inactive, enzyme.
HORMONE ACTIVATION OF ADENYLATE CYCLASE
(1) Peptide hormones are tansported in the blood. (2) They combine with receptors in the plasma membrane of a target cell. (3) The hormone-receptor combination is coupled by G-protein to activate adenylate cyclase. (4) Cyclic AMP then activates one or more enzymes which (5) Phosphorylates specific cellular proteins that (6) alter the activity of the cell in some way.
GTP BINDING PROTEINS
The trimeric GTP binding proteins (G-proteins) play a pivotal role in the signal transduction pathways for numerous hormones and neurotransmitters. The three subunits of the protein are labelled alpha, beta, and gamma. Both the alpha and gamma subunits are bound to the memebrane via attached lipid molecules (related to fatty acids and cholesterol). The receptors are proteins with seven transmembrane alpha-helices.
The binding of the hormone or neurotransmitter to the receptor causes GTP to replace GDP on the alpha subunit. As a result, the alpha subunit dissociates from the other two. Subsequently, the alpha subunit and the combined beta and gamma subunits move along the inner surface of the membrane to specific ion channels or membrane enzymes. Ion channels opened in this way are often potassium channels and an example of a membrane enzyme with this type of activation is adenylate cyclase. These sequence of events ends when the alpha subunit hydrolyzes the GTP to GDP, allowing the subunits to associate again.
Peptide Hormone Signal Transduction via a G Protein-Linked Receptor
REFERENCES
1. http://www.mpi-dortmund.mpg.de/departments.dep1/gtpase/gtpase.php3
2. http://www.hhmi.swmed.edu/Labs/ss/page_2/heterotrimer/Galpha_subunits/index.html
3. http://www.bch.bris.ac.uk/staff/gar/Hormone_Tutorial/intrinsic_gtpase_activity.htm
4. http://www.courses.washington.edu/conj/gprotein/gprotein.htm
5. http://www.entochem.tamu.edu/Teaching/G-Protein/GproteinMovieText.html
6. Mathews, Christopher K., K. E. van Holde,
and Kevin G. Ahern. Biochemistry.
Addison Wesley Longman: New York, 1999.
7. Solomon, Eldra Pearl., Linda R. Berg, Diana W. Martin, and Claude Villee. Biology. Saunders College Publishing. 1996.